Which element will react most vigorously with water, and what is the reaction that occurs?

(a) Lithium; 2 Li(s) + 2 H₂O(l) → 2 Li⁺(aq) + 2 H^–(aq) + H₂O₂(aq)

(b) Potassium; 2 K(s) + 2 H₂O(l) → 2 K⁺(aq) + 2 OH^–(aq) + H₂(g)

(c) Magnesium; Mg(s) + 2 H₂O(l) → Mg²⁺(aq) + 2 OH^–(aq) + H₂(g)

(d) Barium; Ba(s) + 2 H₂O(l) → Ba²⁺(aq) + 2 H^–(aq) + H₂O₂(aq)

Video transcript

All right. Hi, everyone. So this question says to consider the following elements, calcium rubidium, strontium and lithium, which will react most violently with what write the balance reaction equation that occurs. So here we have four different answer choices representing one metal each as well as one reaction equation each. So the first thing I want to point out here is that the four metals listed in our choices are metals that belong to both groups one A and two A. And the reason why I want to bring this up is because recall metals in groups one A and two A are known to react vigorously with water. And when that happens, they combine together to produce hydrogen gas, the corresponding metal ion and hydroxide ion. So recall also that in group one a of the periodic table, the metals that are known to react with water are lithium, sodium, potassium, rubidium and cesium. Whereas in group two A, the metals known to react with water are magnesium, strontium, excuse me, calcium and then strontium as well as bury it. Now for groups one A and two A, it just so happens that as you go down the group, the reactivity of a given metal with water actually increases. And part of the reason why that is the case is because as you go down a group, the ionization energy decreases, meaning that it is easier to remove an electron from a given atom. So of the group won a medals recall that the reaction of either rubidium or caesium with water is considered to be the most violent right to the point where they tend to react almost explosively with water. So definitely dangerous to do. By contrast, right, the reaction of lithium produces some bubbling sodium releases some heat during the reaction. And potassium does actually cause the hydrogen gas produced to burst into flames. But rubidium and cesium are some of the most violent still in group one. A at least now of the medals listed here in group two A. The reaction of barium with water is the only one considered to occur vigorously at room temperature because the reactions of calcium or strontium with water actually occur relatively slowly and the reaction of magnesium with water requires a very high temperature of about 100 °C. So of the metals listed in our answer choices. Rubidium is the only one that reacts most vigorously, right. So rubidium is going to be our answer. So now we can proceed with writing our balanced reaction like so my first rubidium metal which is a solid combines with liquid water to produce rubidium ion or RB positive an aqueous solution as well as hydroxide ion, an aqueous solution. And lastly hydrogen gas. Now, as a quick side note recall that because rubidium is a metal in group one, a its corresponding ion is going to have a charge of positive one. So now let's discuss the balancing of this reaction, right? Because the first thing I want to point out here is that the number of hydrogens on both sides of the reaction arrow are actually unbalanced, right? Because there are two hydrogens on the left side, but there are three on the right side. So to fix this, I'm going to add a stoichiometry coefficient of two in front of both hydroxide and water. Why? Because by doing so, I end up with four hydrogen atoms on either side of my arrow as well as two oxygens on either side. So now that my atoms are balanced, recall that charges have to be balanced as well, meaning that both sides of my equation should be neutral. And if I consider the ions on the right side, I have two negatively charged ions and only one positively charged ion. So to fix this and make my charges balanced, I'm going to add two equivalents of my cion, which in this case is rubidium isle because by doing that, I have equal amounts of positive and negative charges on the right side. So this means that I have to add a coefficient of two in front of rubidium metal. And with that, I've balanced my reaction, that's two equivalents of rubidium solid with two equivalents of water. They yield two equivalents of rubidium ion, two of hydroxide and one equivalent of hydrogen gas. So our answer here is going to be option B in the multiple choice. And with that being said, thank you so very much for watching and I hope you found this helpful.